It's locked up inside Capri's only slammer (6)
I believe the answer is:
prison
'slammer' is the definition.
(I've seen this in another clue)
'it's locked up inside' is the wordplay.
'it's locked up' becomes 'imprisoned' (I've seen this before).
'inside' indicates the central letters.
The central letters of 'imprisoned' are 'PRISON'.
'capri's only' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for prison that I've seen before include "punishment" , "Place of detention for criminals" , "cage" , "Jail" , "porridge" .)